1. Field
The present systems, methods and apparatus generally relate to cryogenic cycle refrigeration systems and in particular to the use of such refrigeration systems in the field of superconducting quantum computing.
2. Refrigeration
Temperature is a property that can have a great impact on the state and evolution of a physical system. For instance, environments of extreme heat can cause even the strongest and most solid materials to melt away or disperse as gas. Likewise, a system that is cooled to cryogenic temperatures may enter into a regime where physical properties and behavior differ substantially from what is observed at room temperature. In many technologies, it can be advantageous to operate in this cryogenic regime and harness the physical behaviors that are realized in the realm of cold. The various embodiments of the systems, methods and apparatus described herein may be used to provide and maintain the cryogenic environments necessary to take advantage of the physics at cold temperatures.
Throughout this specification and the appended claims, the term “cryogenic” is used to refer to the temperature range of 0 to about 93K. A variety of technologies may be implemented to produce an environment with cryogenic temperature, though a commonly used device that is known in the art is the dilution refrigerator. Dilution refrigerators can even be used to achieve extreme cryogenic temperatures below 50-mK. In the operation of a typical dilution refrigerator, the apparatus itself requires a background temperature of about 4K. Thus, the apparatus is typically immersed in an evaporating bath of liquid helium-4 (“4He”) to provide the ˜4K background. The 4He bath may be contained in a vacuum chamber or dewar. By pumping the gas out of the dewar, a high vacuum level may be realized above the surface of liquid 4He and a temperature of around 1K can be achieved. Similarly, if the 4He is substituted with 3He, a temperature of approximately 0.3K can be achieved. The dilution refrigerator apparatus may comprise a series of heat exchangers and chambers that allow the temperature to be lowered further to a point where a mixture of 3He and 4He separates into two distinct phases and pure 3He can move into a mixture of 3He and 4He in a process analogous to evaporation, providing cooling and allowing a temperature of around 10-mK to be achieved. Full details on this dilution effect and the operation of typical dilution refrigerators may be found in F. Pobell, Matter and Methods at Low Temperatures, Springer-Verlag Second Edition, 1996, pp. 120-156.
In conventional dilution refrigerator (“CDR”) designs, mechanical pumps and compressors, and an external gas-handling system, are used to circulate 3He such that it is warmed from the lowest temperature in the fridge up above cryogenic temperatures and towards room temperature before it is returned to the low temperature. The pumps and compressors used are large, expensive, noisy, in need of periodic maintenance, and they inevitably add contaminants to the helium that can plug fine capillaries in the dilution refrigerator, causing problems with reliability. Filters and cold traps can be used to reduce the frequency of plugging, but these systems remain susceptible to contamination due to the smallest leaks. Plugging often requires a complete warm-up of a CDR in order to remove the contaminants and restore the fridge to normal operations. The procedure of warming and subsequently cooling back down to operating temperatures can take several days. CDRs are large, complex, composed of many pieces that are connected by numerous hoses and wires, and they require an elaborate external gas handling system for the circulation of 3He. The various embodiments described herein address these issues to provide improvements to typical CDR designs.
Pulse Tube Cryo-Coolers
Pulse tube cryo-coolers (“PTs”) are devices that may replace the liquid helium evaporating bath in CDRs to provide the initial cooling of ˜4K. A typical PT provides cooling power by closed-cycle compression and expansion of helium. Examples of commercially-available PTs include those made by Cryomech, Inc. of Syracuse, N. Y. Examples of commercially-available pulse tube dilution refrigerators (“PTDRs”) include those made by Leiden Cryogenics BV of Leiden, the Netherlands and those made by VeriCold Technologies GmbH of Ismaning, Germany. While these PTDR systems eliminate the need to consume helium by way of open loop evaporation, they are still expensive, large, complex, multi-piece, multi-connection systems that require an external gas handling system to bring the helium above cryogenic temperatures in providing the required circulation needed for cooling. Due to this complexity, they are prone to leaking and plugging, and require routine and unexpected maintenance of many of the components.
Cryogenic Cycle Dilution Refrigerator
In cryogenic cycle dilution refrigerator (“CCDR”) designs, the external gas handling system used to circulate helium in CDRs is not required. The elimination of the external gas handling system can reduce the size, complexity, and maintenance requirements of a dilution refrigeration system. CCDRs operate by using at least one adsorption pump to circulate helium without ever warming the helium above cryogenic temperatures. The adsorption pumping technique takes advantage of the tendency of gas to condense or adsorb on cold surfaces and be released again in liquid form under the influence of gravity, and to “desorb” from the cold surface when said surface is warmed. By incorporating a pulse tube cryo-cooler instead of a liquid helium dewar, a cryogenic cycle pulse tube dilution refrigerator (“CCPTDR”) can be made. This device can be made much smaller, simpler, cheaper and more reliable than typical PTDRs.
The adsorption pumps that can be used to build a CCPTDR are inherently “single shot” devices, meaning that they can pump for a while but then need to be regenerated before they can pump again. An adsorption pump is regenerated by warming it up, and thereby causing the gas (helium) that it has adsorbed to be released. In order to provide continuous cooling, multiple adsorption pumps can be used in such a way that when one or more of the adsorption pumps is/are pumping, one or more others can be regenerating. In this way a continuous cryogenic cycle pulse tube dilution refrigerator (“CCCPTDR”) can be realized. As a self-contained system, a CCCPTDR can be advantageous in many applications because it is more compact, less complex, and more reliable than alternative cryogenic refrigerator designs, such as the CDR and the PTDR.
Quantum Processor
A computer processor may take the form of an analog processor, for instance a quantum processor such as a superconducting quantum processor. A superconducting quantum processor may include a number of qubits and associated local bias devices, for instance two or more superconducting qubits. Further detail and embodiments of exemplary quantum processors that may be used in conjunction with the present systems, methods, and apparatus are described in US Patent Publication No. 2006-0225165, US Patent Publication No. 2008-0176750, U.S. patent application Ser. No. 12/266,378, and U.S. Provisional Patent Application Ser. No. 61/039,710, filed Mar. 26, 2008 and entitled “Systems, Devices, And Methods For Analog Processing.”
A superconducting quantum processor may include a number of coupling devices operable to selectively couple respective pairs of qubits. Example of superconducting qubits include superconducting flux qubits, superconducting phase qubits, superconducting charge qubits, and superconducting hybrid qubits. Examples of devices that may be implemented as superconducting qubits and/or superconducting coupling devices include rf-SQUIDs and dc-SQUIDs. SQUIDs include a superconducting loop interrupted by one Josephson junction (an rf-SQUID) or two Josephson junctions (a dc-SQUID). The coupling devices may be capable of both ferromagnetic and anti-ferromagnetic coupling, depending on how the coupling device is being utilized within the interconnected topology. In the case of flux coupling, ferromagnetic coupling implies that parallel fluxes are energetically favorable and anti-ferromagnetic coupling implies that anti-parallel fluxes are energetically favorable. Alternatively, charge-based coupling devices may also be used. Other coupling devices can be found, for example, in US Patent Publication No. 2006-0147154 and U.S. patent application Ser. No. 12/017,995. Respective coupling strengths of the coupling devices may be tuned between zero and a maximum value, for example, to provide ferromagnetic or anti-ferromagnetic coupling between qubits.
Superconducting Processor
A computer processor may take the form of a superconducting processor, where the superconducting processor may not be a quantum processor in the traditional sense. For instance, some embodiments of a superconducting processor may not focus on quantum effects such as quantum tunneling, superposition, and entanglement but may rather operate by emphasizing different principles, such as for example the principles that govern the operation of classical computer processors. However, there may still be certain advantages to the implementation of such superconducting processors. Due to their natural physical properties, superconducting processors in general may be capable of higher switching speeds and shorter computation times than non-superconducting processors, and therefore it may be more practical to solve certain problems on superconducting processors.
According to the present state of the art, a superconducting material may generally only act as a superconductor if it is cooled below a critical temperature that is characteristic of the specific material in question. For this reason, those of skill in the art will appreciate that a computer system that implements a superconducting processor, such as a superconducting quantum processor, may require a refrigeration system for cooling the superconducting materials in the system.
In the known art, superconducting computer systems that incorporate superconducting processors and/or superconducting quantum processors have primarily been implemented in the contexts of academia and in-house research and development. Such implementations are generally realized in facilities that can accommodate the size, expense, complexity, and high maintenance demands of typical CDRs and PTDRs. However, it may be advantageous for a provider of a superconducting computer system to endeavor to reduce the expected maintenance demands of the refrigeration system and to consider the potential facility limitations of a recipient of the system. For instance, a recipient of a superconducting computer system may prefer to operate the system within an existing server room or, in general, within a room with limited space that cannot easily accommodate, for example, an external gas handling system.